Am i overreacting for refusing to babysit my sister`s kids after what she said at dinner?

my sister has 3 kids under 10 and somehow over the last few years i became the default backup parent. i work from home, so everybody decided that means im always free. babysitter cancels? she calls me. kid has a fever? she calls me. sometimes she doesnt even ask anymore, she just texts dropping them off in 20 like im a daycare she already pays for or something. i didnt push back much because i love my nieces and nephew and honestly her husband is useless half the time unless somebody is actively watching him parent. plus after my miscarriage last year i think i kind of threw myself into helping people so i didnt have to think too much about my own life. last sunday we were having dinner at my parents place and my mom joked that i should start charging daycare rates. everybody laughed except my sister. she goes well its not like she has a real job anyway, she just sits at home on her laptop all day. i laughed at first because i thought she was kidding but she kept going. saying she has the actual exhausting life because shes raising kids while i just answer emails all day with no husband, no kids, no responsibilities. it got SUPER quiet after that because everybody at the table knows about the miscarriage. even my dad looked uncomfortable and immediately tried changing the subject. i just grabbed my stuff and left because i could literally feel myself starting to cry and i wasnt about to do that in front of everybody over some boxed wine and dry chicken parm. since then ive stopped helping completely. no babysitting, no emergency pickups, nothing. now shes freaking out because apparently childcare keeps falling through and she has to miss work sometimes. yesterday she called saying im punishing innocent kids over one stupid joke and that she was clearly kidding. but honestly it didnt feel like a joke. it felt like one of those things somebody has been thinking for a LONG time and finally let slip. now my mom wants me to let it go because the kids are stressed and my sister is overwhelmed. my dad says he gets why im hurt but thinks im dragging it out too long. part of me feels guilty because the kids didnt do anything wrong, but another part of me feels like ive basically been free labor for years and the second i stop being useful suddenly im selfish and dramatic. idk anymore maybe im overreacting but im still really pissed about it. AIO?